This invention relates to a sensor for thermal mass flowmeters used to measure thermal effects in liquid or gas streams. The sensor includes a plate-shaped thin-film resistor and a holder in the shape of a cylindrical rod. The cylindrical rod includes an electrically insulating material provided with capillary drillings for the conductors which are connected to the thin-film resistor.
In industrial processing engineering, thermal mass flowmeters operate according to the principle of hot-wire or hot-film anemometers. Such flowmeters are known and are used to a large extent, for example, to measure gas velocities in furnaces, smokestacks, electrostatic precipitators or air ducts. A typical mass flowmeter is described, for example, in German Published Patent Application DE-OS 3,542,788, which is entirely incorporated herein by reference.
In addition, thermal flowmeters and hot-wire or hot-film anemometers are described in Kirk-Othmer, Encyclopedia of Chemical Technology, Third Edition, Vol. 10, pages 624-627 and the Supplement Volume, pages 492-493, which are also entirely incorporated herein by reference.
The thin-film resistors used as sensors are provided with a holder which is typically a rod-shaped multi-capillary. The holder also receives the connecting wires of the resistor. The thin-film resistor which includes a ceramic support and a thin metal layer, and optionally a ceramic cover plate, is inserted into a slit in the multi-capillary and fastened with glass solder or a glass frit, as described in DE-OS 3,542,788.
This manner of fastening the resistor into its holder is mechanically unstable. Moreover, with this type of fastening, the glass solder displays a relatively large surface to the environment to be measured. When the sensor is used in an aggressive media or environment, this may result in a rapid decay of the glass solder, and thereby a rapid deterioration of the fastening joint itself.
A further disadvantage of this "massive" fastening type joint is that parasitical heat flows from the heated resistor to the more distant areas of the holder which do not receive good flow contact. Another disadvantage is the relatively high effective heat capacity of the capillary. As a result of these two characteristics, rather long response times occur when changing the medium or the heat resistor temperature, during which times the entire temperature distribution in and around the holder has to be reestablished.